A conventional rotary line device, also referred to as a winch, includes a support structure that is attachable to a recovery vehicle. A winch drum is rotatably mounted on the support structure, with a winch cable or rope being attached to the winch drum and wound about the winch drum in multiple layers. A reversible winch motor is mounted on the support structure for rotating the drum, with a speed reduction transmission connected between the winch motor and the drum. A normally-engaged, releasable drum brake assembly also is mounted on the support structure and connected to the winch drum to stop drum rotation.
A control system is operable to release the drum brake and operate the winch motor in the appropriate direction to pay out or pull in the winch cable as needed. Typically, the winch motor is a single or dual displacement reversible hydraulic motor, and the control system likewise is hydraulic because hydraulic systems can provide high power but are relatively uncomplicated and easy to maintain and service. Electric winch motors and control systems alternatively may be employed.
FIGS. 1A and 1B are simplified schematics that illustrate the general forces applicable to winch operation. FIGS. 1A and 1B each depicts a winch 10 including a drum 12 positioned on a support structure 14. A rope is wound around the drum 12, which would be connected at the unwound end to a load to be pulled. In FIGS. 1A and 1B, the distances r and r′ represent an effective radius from the center of the drum to the outer edge of the wound rope that currently is wrapped around the drum. In practice, the unwound portion of the rope would then be connected at the rope end to a load. Referring to FIG. 1A in particular, for a load F representing a load to be moved by the winch, the torque T experienced by the winch from the resistance force of the load is equal to the load multiplied by the distance r constituting the distance from the center of the drum to the outer edge of the wound rope, such that T=F*r. Referring to FIG. 1B, as the rope is wound resulting in longer radial distance r′ to the outer edge of the wound rope, the effective lever arm of the winch system increases, which increase the torque on the winch drum to an amount T′. Because the load F remains the same, F=T/r=T′/r′. The maximum torque experienced by the drum, therefore, occurs when the rope essentially is fully wound around the drum.
In conventional rotary line devices, such as the described winch, the components, and particularly the support structure and winch rope, are designed and constructed to exert and withstand desired maximum pulling tension and torque forces, essentially the forces experienced when the rope is fully wound. Such maximums typically are substantially greater than the pulling force actually required to pull a load when the rope is unwound within a typical range of usefulness. Relatedly, in a single displacement hydraulic motor, the maximum hydraulic fluid flow and pressure differential across the motor are likewise constant and set based on such maximum requirements, resulting in the maximum motor torque and motor speed also being constant based on the desired maximum capabilities of the winch.
In operation of a winch and associated winch motor, therefore, as the number of layers of winch cable or rope wound about the axis of the drum increases from being wound, the load “seen” by the winch motor increases. This is because the mechanical advantage against the winch increases by virtue of the increase in length of the effective lever arm by adding layers of wound rope. The result can be that the winch can no longer pull the load because with each successive layer of rope that forms on the drum, the pulling force proportionally decreases. For conventional hydraulic winch motors, for which the motor typically has a constant pressure applied, it is not unusual for a winch to lose 40% of the pulling force by as little as the fifth layer of wound rope. Thus, increased torque from the motor above that when wrapping the first layer of cable is required to counteract the proportional decrease in the pulling force as each successive layer of cable wraps around the drum. As a result, the winch components must be designed to withstand the greatest pulling force imposed by the motor when only a single layer of cable is present, even though this greatest pulling force is substantially greater than the force actually produced on successive layers of cable.
Accordingly, a conventional winch is designed so as to accommodate a rope size and structural integrity sufficient for the maximum line pull produced with the first rope layer. To meet this need, conventional retrieval winches and similar rotary line devices are relatively large physically to meet the greatest pulling force requirements. It is desirable, however, to mount such retrieval winch devices onto a vehicle of relatively modest size (e.g., pickup truck, SUV, light truck or car) in which space is at a premium. It has been difficult, therefore, to balance the need for a large winch device to meet the greatest pulling force requirements with a small size for vehicle mounting, while still practically having sufficient power for typical usages.
In conventional winches, the line pull force on the cable or rope is a function of motor torque and the drum diameter that is largely influenced by the number of layers of cable or rope that are wrapped around the drum. Thus, for a given motor torque or current, the available line force is dependent on the number of layers of rope or cable that are wrapped on the drum.
The accepted practice for rating winches for rated load is the maximum pull force on the bottom layer of rope or cable that is wrapped around the drum. Conventional methods for limiting the load of hydraulic winches to prevent rope breakage indirectly limit the load using a pressure relief valve. This results in reduced rated load on subsequent layers due to increased torque on the drive motor therefore reaching the relief pressure at lower and lower loads proportional to the layer.
Historically, one alternative is to use a traction winch with a separate storage drum adding both weight and expense.